1. Field of the Invention
This invention relates to a linear alternator for use in generating an alternating current and more particularly to a relatively inexpensive electromagnetic device of low reciprocating mass for generating an alternating electrical current without the use of permanent magnets.
2. Description of the Prior Art
Many methods have been devised to generate an alternating current by converting mechanical energy into electrical energy via electromagnetic principles. The basic principle requires the relative motion of an electrical conductor, usually a conductive winding, with respect to a magnetic circuit to induce the flow of an electrical current in the conductive winding. Most devices employ rotational motion of an electrical conductor with respect to a multipole magnetic circuit as a means to induce an alternating electrical current. If a reciprocating member is used to provide the relative motion, significant waste and inefficiencies of translational to rotational motion conversion are experienced. Accordingly, few alternators have adopted a translating or reciprocating member to directly impart the relative motion.
Those prior art alternators that have adopted a translating or reciprocating member are generally referred to as linear alternators. Linear alternators are often used with free-piston stirling engines to extract electrical power from the reciprocating elements. In these prior art linear alternators, permanent magnets are utilized to develop a magnetic flux field which flows through the magnetic circuit. This flux field is traversed by an electrical conductor, usually in the form of an inductive coil. There are some general advantages to this approach. Linear alternators equipped with permanent magnets are physically smaller, require less volume, and tend to weigh less than alternative approaches.
However, permanent magnet linear alternators do have major disadvantages. The cost of the permanent magnets used to develop the magnetic flux field can represent up to 35 percent of the total alternator assembly cost. Also, the alternating current generated by a permanent magnet linear alternator is difficult to control precisely. This disadvantage arises from the fixed magnetic field flux (or B-field) that permanent magnets develop. As an alternative to modulating the B-field, the stroke of the linearly reciprocal member, usually affixed to the engine piston, must be adjusted to vary the induced current. Such adjustment is not always convenient.
Alternatives to permanent magnet linear alternators which eliminate the permanent magnet do exist and employ a B-field generator incorporated into the reciprocating elements to provide the required alternating magnetic flux circuit. In such applications, the alternating current generator is generally used to increase the gain of an alternating current input electrical signal. In this approach, a pickup coil is mounted on the reciprocating element to generate an AC voltage and an exciter coil is mounted on a proximate stationary element to provide power to the reciprocating field pickup coil. This AC voltage is then rectified to a DC voltage by diodes mounted on the reciprocating element. The DC voltage, in turn, generates a B-field through a field coil mounted on the reciprocating element. A stationary inductive coil mounted within this B-field then produces an alternating current.
A major disadvantage of this approach to linear alternators is that the reciprocating element must include the field pickup coil, the diode assembly, and the field coil. These elements, especially the field coil, significantly increase the mass of the reciprocating element. In addition to higher inertial loads, the additional mass must be compensated for by the use of larger stirling engine pneumatic bounce springs. In the case of a free-piston stirling engine (a common motive source of linear alternators), larger pneumatic bounce springs tend to reduce mechanical efficiency. The additional mass of the components also tends to increase the complexity of the reciprocating element, which must function within extremely tight tolerances to reduce gas leakage.